#include <stdio.h>

int main()
{
	int n = 0;
	int sum = 0;
	int flag = 0;
	int value = 1;
	scanf_s("%d", &n);
	for (int i = 1; i <= n; i++)
	{
		sum += value;
		flag++;
		if (flag == value)
		{
			value++;
			flag = 0;
		}
	}
	printf("%d\n", sum);


	return 0;
}